When drilling for oil and gas it is desirable to be able to steer the drill bit, i.e. to move the drill bit in a chosen direction, so that the drill bit does not have to follow a path determined only by gravity and/or the drilling conditions.
One method of steering a drill bit is to use a downhole assembly with a bent housing, the bent housing resulting in the leading end of the drill bit being offset from the longitudinal axis of the majority of the drill string along the required toolface. The downhole assembly includes a downhole motor connected to the drill bit by way of a flexible driveshaft which rotates within the bent housing, the motor driving the drill bit to rotate whilst the remainder of the drill string, including the bent housing, does not rotate. One such downhole motor is a mud motor which uses the flow of drilling mud to drive the drill bit. The bent housing allows the drill bit to follow a non-linear or curved path, the drill bit moving in the direction of the offset.
Often, this method and apparatus will be utilised when the desired direction and degree of curvature of the borehole is known. However, to cater for unexpected drilling conditions an operator will usually design the housing with a greater bend than necessary, so that the desired degree of curvature can be achieved even if the drilling conditions result in the drill bit deviating from a linear path less than was expected. If, however, the drill bit does deviate as much as expected, this will result in the curvature of the borehole exceeding that desired, so that a linear (or more linear) length of borehole needs to be drilled to compensate. The linear (or more linear) length of borehole is drilled by rotating the whole of the drillstring, which rotates the bent housing and thereby continuously changes the direction of the offset of the drill bit and cancels out the tendency to curve in one direction.
Accordingly, the use of such a method requires the drill string to be non-rotating whilst a curved borehole is being drilled. It is widely recognised that a non-rotating drill string experiences greater friction upon the borehole wall than a rotating drill string, i.e. the resistance to drill bit advance will comprise the resistance of the rock through which the drill bit is moving, plus the resistance to movement of the drill string along the borehole. A drill string which is rotating experiences less resistance to movement along the borehole and therefore enables the drilling of deeper boreholes. Very deep boreholes are commonly required to reach the remaining reserves of oil and gas, and those reserves cannot all be reached with a non-rotating drill string.
In addition, a rotating drill string is less likely to buckle under the applied axial load than a non-rotating drill string. Furthermore, borehole cleaning is improved with a rotating drill string, i.e. drill cuttings in the drilling fluid returning to the surface are less likely to sink and settle at the low side of a (non-vertical) borehole.
A steering assembly and component is described in U.S. Pat. No. 7,270,198. This document describes a downhole assembly having a downhole motor connected to a drill bit by way of a flexible drill shaft which can rotate within a bent housing. A clutch mechanism is located between the drill shaft and the bent housing, the clutch mechanism being able to drive the bent housing to rotate so as to change the toolface offset and permit the drilling of a linear (or more linear) borehole. A set of spur gears is provided between the clutch mechanism and the bent housing, which gears are stated to reduce the rate of rotation of the bent housing relative to the rate of rotation of the drill shaft. This document utilises a non-rotating drill string in the form of a continuous pipe, and therefore shares the disadvantages of the other known non-rotating drill string methods described above, so that the described assembly and component are only useful for relatively short-length boreholes such as those provided for the underground utilities to which the document is directed.
British patent applications 2 435 060 and 2 440 024 each disclose a steering assembly for a drill bit, the downhole assembly comprising a mud motor connected to the drill string, a bent housing connected to the mud motor and a drill bit connected to the bent housing. The mud motor is connected to the drill string by way of a slipping clutch mechanism, the torque which is transmitted by the clutch being variable so as to match the counter-rotation torque experienced by the mud motor as the drill bit rotates. The slipping clutch mechanism must be designed to withstand the considerable torque which can be imparted by the mud motor, a typical drilling torque for a 9.625 inch mud motor being 20,000 lbf. ft, (and a maximum torque being around 32,000 lbf. ft.). In addition, the slipping clutch mechanism must be able to react to rapid and significant changes in the instantaneous torque as the drilling conditions change. The apparatus of these patents is therefore highly complex and expensive.
Other steering apparatuses and methods are known, for example the steering component described in our published European patent application EP-A-1 024 245. That steering component allows the drill bit to be moved in any chosen direction, i.e. the direction (and degree) of curvature of the borehole can be determined during the drilling operation, and as a result of the measured drilling conditions at a particular borehole depth. That steering component, as with the steering assemblies of the two identified British patent applications, can be used with a rotating drill string and therefore avoids the disadvantages of the first cited document. Despite the advantages of these steering components and assemblies, however, operators require a less complex steering component and assembly for many applications.